In wireless communication systems, transmission techniques involving multiple antennas are often categorized as open-loop or closed-loop, depending on the level or degree of channel response information used by the transmission algorithm. Open-loop techniques do not rely on the information of the spatial channel response between the transmitting device and the receiving device. They typically involve either no feedback or the feedback of the long term statistical information that a base unit may use to choose between different open loop techniques.
Closed-loop transmission techniques utilize knowledge of the channel response to weight the information transmitted from multiple antennas. To enable a closed-loop transmit antenna array to operate adaptively, the array must apply transmit weights derived from channel state information (CSI) between each of the transmitter's antennas and each of the receiver's antennas which may include the channel response, its statistics or characteristics, or a combination thereof. One method to obtain the CSI is through a feedback channel between the receiver (e.g., a mobile station (MS) or User Equipment (UE)) and the transmitter (e.g., a base station (BS)). This CSI feedback channel may consist of any technique known in the art such as analog feedback of the channels, analog feedback of the statistics (for example, the covariance matrix or the eigenvector/eigenvectors), quantized feedback of the statistics, quantized feedback of the channel, or codebook feedback. Another method to obtain the CSI in Time Division Duplex (TDD) systems is to leverage multipath channel reciprocity and calculate the necessary CSI from prior transmissions received from the intended receiver.
When the CSI needed for closed-loop transmission is obtained through a feedback channel, the transmitter must have a mechanism that enables the receiver to estimate the channel between the transmitter's antennas and the receiver's antennas. The channel estimation between the transmit and the receive antennas is also needed for the calculation of non-spatial feedback information including modulation and coding rate (MCS), sub-band selection that are applicable for both open-loop and closed-loop transmissions. The usual mechanism to enable the channel estimation by the receiver is by the transmitter sending pilot signals (also known as reference symbols) from each of the transmit antennas which essentially sound the channel. A pilot signal is a set of symbols known by both the transmitter and receiver. The mobile would then use the pilot signal to compute channel estimates which can then be used to determine the CSI feedback.
In a TDD system, when the CSI needed for closed-loop transmission is obtained by leveraging multipath channel reciprocity, the mobile may send a specialized sounding waveform that allows the BS to calculate the uplink (UL) channel. The downlink channel between the BS transmit antennas and the MS receive antennas is then estimated under the assumption of reciprocity. In some cases, a specialized sounding waveform is not needed and the BS can simply leverage prior UL transmissions (e.g., data or control transmissions).
For example and referring now to FIG. 1, a time-frequency diagram 100 of an Orthogonal Frequency Division Multiplexing Access (OFDMA) frequency bandwidth is provided that illustrates a distribution of partial usage of subchannels (PUSC) major group distribution within the OFDMA frequency bandwidth. A vertical scale of time-frequency diagram 100 depicts multiple blocks of frequency (frequency subcarriers) of the frequency bandwidth. A horizontal scale of time-frequency diagram 100 depicts multiple blocks of time, such as time slot intervals 101-103, of a sub-frame that may be allocated. A major group scheduling period, also known as major group time-stripe and typically comprising one frame, in turn comprises multiple time slot intervals, each time slot interval comprising one or more time slots.
A WiMAX 20 MHz (megahertz) bandwidth comprises 120 physical clusters, where a cluster comprises 14 consecutive subcarriers, that is, 12 data subcarriers and 2 pilot subcarriers, over an OFDMA symbol. The 120 clusters are renumbered to form logical clusters, and the logical clusters are grouped to form multiple major groups. A WiMAX downlink (DL) then comprises six major groups, such as major group 1, major group 2, and major group 3 of time-frequency diagram 100, each major group comprising 6 or 8 logical clusters. Subcarriers within a major group are organized into subchannels, where a subchannel forms a minimum unit of allocation. A user, or user equipment (UE), then is assigned to receive a data transmission on one or more subchannels within a major group.
The pilots of the major group are distributed across the frequency bandwidth of the major group and are used by the UE to estimate the channel between the base station (BS) and the UE to enable the UE to demodulate the transmitted data. To enable closed loop transmission on the DL, the UE also may monitor the pilots of the assigned major group and may feed back, to the BS, channel state information (CSI) that is determined based on measurements of the pilots, for example, statistical feedback such as a covariance matrix or any other type of matrix, eignevectors, channel quality mean and variance, a received signal quality information, a channel frequency response, or any other type of channel feedback known in the art. On the other hand, in TDD where reciprocity is leveraged, the CSI for enabling closed-loop DL transmissions can be determined from UL transmissions such as sounding, data transmissions or control transmissions. Then, based on the CSI, the BS may determine optimal beamforming weights in order to beamform DL signals for transmission to the UE over the major group.
In the IEEE 802.16-2009 standard for OFDMA transmission with the PUSC subchannelization, the pilots of a major group are shared amongst all subchannels that belong to the major group. All UEs that are assigned to receive data on subchannels within the major group leverage the same set of pilots to demodulate their received data. In a TDD deployment where adaptive closed-loop transmission is enabled by leveraging reciprocity, the pilots within a major group and within a time-slot interval (see FIG. 1) must be beamformed in the same way as the data so that both pilots and data go through the same aggregate channel. The pilots cannot be “broadcast” or “per-transmit-antenna” in this situation because there is no provision for informing the MS of the transmit weights used on the data transmission. (With broadcast or per-antenna pilots, the UE would need to know both the per-antenna DL channel and the transmit weights.) As a result of this shared pilot constraint, all subchannels of the major group must be beamformed in the same way. As a further result, it is therefore not possible within a major group and within a time-slot to beamform to one user on one subchannel with one set of transmit weights while also beamforming to another user on a different subchannel with a different set of transmit weights. Note that the term “major group time slot” is used to refer to a portion of the time-frequency resources consisting of one major group in frequency and one time slot interval in time. Furthermore, note that the term major group time stripe refers to a portion of the time-frequency resources consisting of one major group in frequency and one entire time stripe in time (where a time stripe refers to a frame consisting of multiple consecutive time slot intervals).
As a result of these constraints, in WiMAX systems using PUSC subchannelization, the transmit weights used to beamform the downlink data must be fixed at least over a major group time slot. Furthermore, if the beamforming transmit weights are adaptively and uniquely calculated for each user, then only one user can be allocated to a major group time slot since the entire major group time slot will be beamformed according to the user's CSI. As a result, an 802.16e-OFDMA system that uses adaptive and unique transmit weights for each user has a minimum allocation constraint of one major group time slot. Unfortunately, with small size packets, this minimum allocation constraint can often result in unassigned (wasted) resources. For example and referring to FIG. 1, in a depicted time-slot interval, DL signals intended for a first UE are transmitted to the UE over major group 1 and beam 1, DL signals intended for a second UE are transmitted to the second UE over major group 2 and beam 2, DL signals intended for a third UE are transmitted to the third UE over major group 3 and beam 3, and so on.
When a UE receives data on the downlink in this situation, these constraints cause additional constraints on the channel estimation algorithm that the UE may use. When estimating the channel for its allocation, the UE generally must only use pilots that go through the same channel as the data it is attempting to decode (otherwise channel estimation performance will be degraded). As a result, in an 802.16e-OFDMA system that uses adaptive and unique transmit weights for each UE, the UE may not use pilots outside of its allocation for channel estimation. Thus, as a result of the above-mentioned constraints, for each major group time-stripe, the per-UE DL channel allocation granularity is an entire major group time slot and each beam is uniquely associated with a single UE. If a user's data fails to consume the entire bandwidth of the major group, for example, where the user traffic consists of lots of small data packets such as in VoIP (Voice over Internet Protocol) transmissions, then the remainder of the major group will remain unused and bandwidth will be wasted. This problem is worsened when a MIMO-Matrix B transmission scheme is used because the bandwidth requirements (i.e., the required time-frequency allocation size) for a data packet of a specified size are even smaller than when a MIMO-Matrix B transmission scheme is not used. In addition, in a closed loop WiMAX system using transmit weights that are adaptively and uniquely computed for the UE, the UE may use only pilots belonging to its allocation for estimating the channel for DL demodulation, which may result in sub-optimal DL channel estimation performance compared to situations where pilots outside of its allocation are usable by the UE.
Therefore, a need exists for an improved system of beamforming and DL scheduling, which does not have the wasted system capacity of the prior art and which provides improved DL channel estimation over the closed loop feedback schemes of the prior art.
One of ordinary skill in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of various embodiments of the present invention. Also, common and well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.